The field of the invention is that of the treatment of untreated water. More specifically, the invention relates to the removal or, at least, to the high abatement of the micropollution contained in untreated water and, particularly, in drilling water.
Untreated water contains little colloidal organic matter and it is generally unnecessary to make it undergo treatment constituted by a succession of steps of the flocculation-decantation-filtration type. On the contrary, this untreated water is generally charged with micropollutants, and may contain microorganisms or again reduced mineral compounds (ferrous iron, manganese etc.).
The removal of the micropollutants can be done, for example, by stripping (forced air circulation) if they are volatile or by adsorption on active carbon or on other types of materials such as ion-exchanger resins or again by physical separation with methods using nanofiltration membranes and reverse osmosis. All these last-named methods may be very efficient but have the drawback of not destroying the pollution but only of holding it back.
It has therefore appeared to be necessary to use oxidizing techniques for the removal or high abatement of the micropollutant content of this untreated water. Although certain prior art techniques consist in using chlorine or chlorine dioxide, ozone has appeared to be the most useful oxidant because it is capable of completely destroying organic matter by mineralizing it into CO.sub.2 and mineral salts under certain conditions of implementation. Ozone is also used for the depollution of water lightly charged with organic pollutants and notably in the context of making consumption water potable. For, the ozone molecule permits radical reactions involving the OH.degree. radical enabling the organic matter to be highly oxidized.
One of the major problems encountered in the context of the treatment of untreated water with a view to making it potable lies in the presence, in this water, of particular organic micropollutants constituted by pesticides, insecticides, fungicides, herbicides and rodenticides. Indeed, these organic compounds cannot flocculate and are difficult to remove by conventional means of oxidation. Since these micropollutants, even in low doses, have high toxicity, one of the priorities of the water treatment is to further their removal to the utmost possible extent.
Among these compounds, we may note for example the frequent presence in drilling water of atrazine and simazine which are herbicides.
Atrazine is constituted by 2-chloro-4-ethylamino-6-isopropylamino-6 s-triazine. The simazine molecule is 2-chloro-4,6-bis(ethylamino-)-s-triazine.
The abatement of these pesticidal compounds during the conventional flocculation-decantation step is very low and, on an average, about 5%. After the operation of fast filtration on sand, the abatement reaches an average of 20%. The use of an ozonization unit after a sand filter using a low treatment rate of the order of 0.85 mg O.sub.3 /.sub.l leads to a final abatement of 30%. This final abatement does not often allow for reaching the prescribed standards for potable water which, in the case of atrazine, is 0.1 .mu.g/l. It is therefore necessary to have recourse to a higher ozone rate. A ozone level of 4 mg O.sub.3 /.sub.l enables the atrazine concentration to be reduced by 60%. Although it is further possible to improve this abatement by further increasing the ozone level injected into the water to be treated, there are then encountered problems of cost and especially problems relating to the removal of residual ozone in the treated water. It is then necessary to extend the contact time of the ozonized water coming out of the ozonization units, for example by making this water pass into contact columns fostering the degassing of this residual ozone. For example, it may be necessary, after a contact time of two minutes in the prior art ozonization units, to extend the contact time for eight minutes by making the ozonized water pass into a large-sized reactor so that the residual ozone contained in them can be essentially removed.
In the prior art, there are known plants for disinfection by ozone provided with contactors with diffusion of ozone by porous structures. Such plants have many drawbacks, due essentially to the deterioration of the pores in the course of time. Furthermore, and as mentioned here above, it is necessary, in order to ensure the minimum contact time necessary for the totality of the liquid to be treated, to maintain a substantial average time of passage in the reactors. This in particular calls for oversized structures that are costly and use space.
Besides, another problem presented by the ozonization reactors using porous structures lies in the fact that they may induce a lack of homogeneity of the ozone concentration in the water to be treated.